Conventionally, a magneto-optical disk memory having a film with perpendicular magnetization, which enables a detection of magnetization direction in the film of the perpendicular magnetization by irradiating laser light on the perpendicular magnetization film to reflect and detecting the respective magnetization directions from the reflected light of the laser light varying due to the polar Kerr effect, has been used as a rewritable magneto-optical recording medium in its practical application.
Such a magneto-optical disk memory suffers from a drawback that readout characteristics deteriorate when a recording bit diameter as a recording magnetic domain and an interval between the recording bits become smaller with respect to a diameter of a light beam of a semiconductor laser converged on the magneto-optical memory.
Such a problem is caused by the following reason. When the laser light is focused on a target recording bit, the adjoining recording bits also fall within the diameter of beam spot, and therefore each recording bit can not be read separately.
In order to solve the above-mentioned problem, Japanese Laid-Open Patent Publication No. 81717/1993 (Tokukaihei 5-81717) discloses a super resolution magneto-optical readout technique. More concretely, as shown in FIG. 22(a) and 22(b), a track 203 is provided in a spiral form or concentric form on a disk-like substrate 200, and a recording bit 201 having a readout layer 3' and a recording layer 4' is formed at a predetermined interval along each track 203.
In this state, laser light 205 is projected onto the readout layer 3'. On a spot 206 on which the laser light 205 is irradiated, the readout layer 3' and the recording layer 4' show temperature distributions corresponding to a light intensity distribution in the direction of a diameter of the laser light 205. Here, the readout layer 3' has an in-plane magnetization at room temperature, and has a perpendicular magnetization in response to a temperature rise.
The polar Kerr effect used as readout means for the magneto-optical recording medium is obtained only from a perpendicular magnetization component of the readout layer 3' whereon the laser light 205 is projected. Therefore, perpendicular magnetization is shown only in the area irradiated with the laser light 205, i.e., the central portion having a temperature rise in the laser beam spot 206 on the readout layer 3', thereby obtaining a readout signal from the readout layer 3'.
As a result, only the magnetization state of the recording layer 4' of the recording bit 201 that is present in the central portion of spot 206 of the laser light 205 is copied to the readout layer 3' by the exchange coupling. While other recording bits 202 of the readout layer 3' show in-plane magnetization, thereby allowing only information on the recording bit 201 to be readout.
For the above-mentioned reason, even if the diameter of each recording bit 201, 202 or the interval between the recording bits 201 and 202 is made smaller than the diameter of the spot 206 of the laser light 205, the recording bits 201 and 202 can be readout respectively, thereby achieving high-density recording of information to be readout.
However, in the conventional arrangement disclosed in the above publication, a further improvement of the recording density of respective recording bits 201 and 202 is restricted by the problem associated with a readout operation.
More specifically, with the prior art, the readout layer 3' generally shows a gradual change from in-plane magnetization to perpendicular magnetization as the temperature increases. Here, since the temperature of recording bit 202 adjoining to the recording bit 201 to be readout is also raised to a degree, the magnetization in the readout layer 3' in the adjoining recording bit 202 faces an intermediate direction in the course of the change from the in-plane magnetization to the perpendicular magnetization, thereby having a component in the perpendicular magnetization direction.
Thus, when reading information recorded on the recording bit 201, the perpendicular magnetization component of the adjoining recording bit 202 is also read by the laser light 205. Consequently, each recording bit 201 cannot be readout by being completely separated from the signal of the recording bit 202.
Accordingly, there is a limitation in terms of reducing the size of the recording bits 201 and 202 and the interval between the recording bits 201 and 202 associated with the readout operation. It is therefore difficult to further improve the recording density of the recording bits 201 and 202.